The invention relates generally to devices for maintaining preload in spaced-apart antifriction bearings, commonly applied to machine tool spindles.
In particular, the invention specifically relates to those spindle bearing preload devices which are adjustable in use, so that the bearing preload level may be varied.
It is commonly known in the machine art to preload the inner and outer races of antifriction bearings against one another to remove the "shake" or clearances, from a set of bearings and to induce a desired amount of strain into machine elements, for example, a tool spindle, so that machining force levels will not overcome the preload and thus cause the shake to be seen as spindle movement. Additionally, a certain amount of strain is often put into the machine tool spindle so that thermal growth may also be accommodated without loosening the bearing sets.
A high level of preload is useful for maintaining spindle stability during cutting operations, but it is also desirable that the preload amount be matched to the cutting forces seen by the spindle. For example, at high rotational spindle speeds, light cutting forces are generally encountered, while at the low end of the spindle rotational speeds, heavy cutting forces are generally seen. Since the preload amount directly influences the torque on a bearing set, it can be seen that unnecessarily large preload forces can cause high torques which, in the case of high rotational speeds, consume a large quantity of horse power. Such high horse power running conditions cause excessive heat buildup and contribute to early failure of the bearings.
Thus, it has been recognized by prior art practitioners that it is desirable to have a spindle bearing preload device which can be varied as to the amount of preload to accommodate higher and lower rotational speeds satisfactorily, at low running temperatures.
Several interesting and pertinent prior art patents discuss the problem and present solutions therefor, to wit: U.S. Pat. Nos. 3,222,991, Dec. 14, 1965; 3,352,611, Nov. 14, 1967; 3,664,718, May 23, 1972; 3,804,477, April 16, 1974; 3,945,694, Mar. 23, 1976; and 4,226,485, Oct. 7, 1980.
Most bearing preload devices act in a similar fashion: front (nearest the spindle nose) and rear spaced-apart antifriction bearings are mounted with their inner races secured to the spindle while the outer race of the front bearing is positioned against a solid housing shoulder and the corresponding outer race of the rear bearing is loaded by mechanical or fluid springs to induce a strain in the spindle as the antifriction elements transmit the preload to the spindle. In this common application, the rear bearing outer race must be slidable in its bore, necessitating radial clearance which, in turn, may contribute to radial vibration or radial loss of precision in the spindle mounting arrangement. In the usual fixed preload system, i.e. where preset mechanical springs thrust the rear bearing outer race in a rearward direction, spindle growth due to thermal excursions may be accommodated while maintaining preload. In these prior art assemblies, machine cutting forces directed toward the spindle nose are borne solidly by the shouldered outer race of the front bearing. However, when the spindle of the machine experiences forces which tend to pull the spindle out of the housing, i.e. in a forward direction, the outer race of the rear bearing sees the pulling load and can resist movement only up to the limit of the spring preload. After this point, some spindle movement may be seen in a forward direction as pulling forces increase.
Applicant has determined that, in an ideal bearing preload device involving spaced-apart antifriction bearings on a spindle, the front bearing outer race should be rigidly backed up by a housing shoulder as with prior art, and that the outer race of the rear bearing should be provided with a means for varying the axial load on the bearing in a uniform fashion. But further, the outer race should be easily movable when increasing or decreasing the preload amount, and yet the assembled device should be free of clearances which may contribute to radial shake of the bearing and a consequent disturbance of the set-up. Additionally, the device should be easily settable throughout a preload range and yet should be resistant to pullout forces exerted against the device, certainly to the extent of loads which exceed the preload amount.
The patented preload devices enumerated above merit discussion:
U.S. Pat. No. 3,222,991 teaches a mechanism for adjustably preloading a rear bearing set for a machine spindle wherein a plurality of pistons mounted in a loading ring directed against the outer race of the rear bearing set serves to provide a variable rearward thrust as hydraulic pressure is varied to the pistons. The loading ring or plunger carries the pistons, and is integrally formed with a sliding member which carries the outer races of the preloadable bearing set. The plunger slides in a cylinder bore having clearance provided therefor, and the plunger has an integral, thin walled annular chamber into which hydraulic fluid may be introduced. When fluid pressure is introduced into the chamber, the chamber expands thereby locking or tending to seize the piston by friction through radial forces applied to the cylinder wall. The radial forces of the expandable chamber create a normal force on the cylinder wall and the resultant axial frictional force serves to resist pullout of the spindle when pulling forces are exerted against the spindle in a "forward" direction. An alternate embodiment of the invention shows that the individual radial array of pistons may be replaced with one central piston coaxial with the spindle axis.
Several difficulties are encountered in the design depicted, for example: plural hydraulic lines must be utilized, i.e. one to supply desired preload pressure to the loading pistons and a second, very high pressure, line must be directed into the annular expansive cavity of the loading plunger. Only a fraction (friction coefficient) of the large radial force is useable to resist spindle pullout. Additionally, radial clearance around the sliding plunger in the cylinder may possibly cause a disturbance in the radial position of the rear bearing set. Since it is preferred to have the stiffest radial mounting possible, together with a stiff axial mount, the assembly presents a hydraulic spring, in effect, for taking up the radial clearance after the plunger is moved. Further, the hydraulic clamp affected by the high pressure line must be relieved when it is desired to change the preload through the lower pressure line.
U.S. Pat. No. 3,352,611 illustrates a spindle bearing preload device wherein the rear set of spindle bearings are thrust rearwardly by mechanical springs loaded against an outer race carrier. The outer race carrier is mounted in a linear ball bushing, so that it may be easily moved in an axial direction. The maximum amount of preload is determined by the spring compression, and the spring force may be overcome by a plurality of fly balls carried at the rear end of the spindle and designed to fly away under centrifugal force. The radial fly away force is directed against a ramp surface on the outer race carrier and is thus transmitted as an axial force which may overcome the spring preload force. Thus, at high speeds, the preload is lessened on the bearings.
The basic mechanism shown has its maximum preload derived from a compressed plurality of axial springs arrayed around the spindle. The fly balls rely on frictional drive to be revved up to proper speed to produce the radial load needed to transform into the axial unloading force. Further, a linear ball bushing of the type shown, i.e. nonrecirculating bearings, requires a close fit between the inner member and outer member bearing on the balls, so that the balls will roll relative to the two. However, in actual practice, it is difficult to maintain parallelism of the inner surface and outer surface which contact the balls, and the balls and ball cage generally tend to creep or walk to one end of the assembly. In such event, the antifriction linear motion would be negated by skidding of the balls.
U.S. Pat. No. 3,664,718 teaches a rear bearing set preload device utilizing a sleeve for carrying the outer races of the rear bearing set. The sleeve has a circular piston face so that variable pressure may be applied to the face to change the preload. The device also teaches that the sleeve is supported in a plurality of hydrostatic pockets so that it may freely move in an axial direction. The device has no locking means for maintaining the preload, and thus the piston and variable hydraulic force create a fluid spring against which any pull-out forces react.
U.S. Pat. No. 3,804,477 illustrates a device for relieving a maximum set-in preload which clamps a pair of spaced-apart bearings together. A thrust face of a nut member is directed against a bearing inner race while the outer races are held to a preset spaced-apart dimension, and the nut member has a diaphragm at its thrust end. A plurality of weights are attached to the diaphragm at an inner chamber formed within the nut clamping member and, at high rotational speeds, the weights will tend to fly away centrifugally and slightly curl the diaphragm backward away from the preloading thrust face of the inner race.
The device requires the use of a very complex nut member which is machined out and, presumably, welded together so that the hollow face of the disc-shaped element is covered with a diaphragm surface. The diaphragm has a plurality of weights affixed around its inner bore and the preload cannot be varied at will during operation. The set preload may be lessened only at different speeds, and only in accordance with the centrifugal forces generated therein. Thus, the preload cannot be tweaked for accommodation of expected cutting forces.
Additionally, the mechanism loads the rear bearing with a mechanical diaphragm spring without a lock or stop, so that any pull out forces on the spindle will pull directly against the mechanical spring force of the preload device.
U.S. Pat. No. 3,945,694 illustrates a spindle preload device wherein the rear outer race carrier of a spindle bearing set is thrust rearwardly by a plurality of mechanical springs which may accommodate thermal expansion and contraction of the spindle. Additionally, the device teaches that the rear bearing race carrier has rows of barrel-shaped rollers, located in a suitable bore, to provide an antifriction rolling assembly so that sliding forces are minimized.
The device is generally complex in that the barrel rollers must be well matched to take up any radial shake or clearance which may exist in the assembly. Further, the maximum force is induced by a plurality of compression springs located in a circular array around the spindle and directed at the thrust face at the rear of the outer race bearing carrier. Thus, the preload in the assembly cannot be varied to suit the loads, but merely the spindle growth is accommodated. Incidentally, spindle growth will cause an increase in the working height of the compression springs, which lessens the spring load. Any pull out forces encountered by the spindle are taken directly against the preload springs at the rear of the spindle.
U.S. Pat. No. 4,226,485 teaches a bearing assembly which embodies certain thermal adapters and teaches thermal philosophies involved for compensating for spindle growth and radial growth of bearing elements. The general idea is to match the bearing elements such as rollers and inner and outer races and controlling the welled-up heat by specially designed thermal barriers at the rear and front bearings so that the bearing elements may not separate and "shake" may not occur. Of interest in the subject patent is the fact that FIG. 1 shows the rear bearing of the assembly mounted in a single diaphragm ring, and FIG. 6 of the assembly shows the rear bearing mounted in a pair of diaphragm rings and having axial loads applied to the outer race by a circular array of axial pistons. While not depicted in the drawings, the specification also relates that the circular array of pistons may be substituted for by a single annular piston centered on the spindle axis.
No locking mechanism is shown in the reference patent, and any pull out loads would be directed against the hydraulic piston or pistons which effectively create a hydraulic spring. Thus, when the spring force is exceeded, the spindle may move forward under a pull out load. Additionally, it should be noted that the assembly teaches that the screws for clamping the rear spindle bearing cover to the housing pass through clearance holes machined through the bearing support diaphragm in both FIGS. 1 and 6. It is evident from good design practice, that a circular pattern of holes through a diaphragm which is expected to flex will tend to create stress risers where failure or crack propagation may occur.
Applicants have obviated the difficulties inherent in the prior art devices by providing a unique spindle bearing preload device which employs an axially-flexible yet radially-stiff element--for example, a circular diaphragm, supporting the rear bearing of an assembly, wherein loading of the diaphragm and subsequent movement of the outer race of the rear bearing will not disturb the centrality of the outer race relative to the spindle axis, since sliding elements or rollers are not used. Additionally, selectively varied hydraulic pressure is directed against the diaphragm face to provide a large operating piston which is concentric with the spindle axis. Still further, applicant's invention has provided for a positive stop face in line with the axially-powered diaphragm support so that at a predetermined movement of the outer race corresponding to a predetermined maximum strain or preload in the spindle, the stop face will be encountered and, at such time, the hydraulic pressure may be raised to such level that an extremely large holding force will result in the set-up without further increase in the preload of the bearing sets. In this manner, therefore, at the most desired time, i.e. at low speeds and relatively high preloads, the holding force may be raised to sufficiently high level to resist any expected pull out, or oppositely-directly force, on the spindle, while the spindle bearings do not suffer the effects of having to run with abnormally high preload force.